RE: [PATCH] Drivers: hv: vmbus: Add cpu read lock

[Michael Kelley (LINUX)]

From: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx> Sent: Wednesday, June 8, 2022 10:27 PM
> 
> Add cpus_read_lock to prevent CPUs from going offline between query and
> actual use of cpumask. cpumask_of_node is first queried, and based on it
> used later, in case any CPU goes offline between these two events, it can
> potentially cause an infinite loop of retries.
> 
> Signed-off-by: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx>
> ---
>  drivers/hv/channel_mgmt.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> index 85a2142..6a88b7e 100644
> --- a/drivers/hv/channel_mgmt.c
> +++ b/drivers/hv/channel_mgmt.c
> @@ -749,6 +749,9 @@ static void init_vp_index(struct vmbus_channel *channel)
>  		return;
>  	}
> 
> +	/* No CPUs should come up or down during this. */
> +	cpus_read_lock();
> +
>  	for (i = 1; i <= ncpu + 1; i++) {
>  		while (true) {
>  			numa_node = next_numa_node_id++;
> @@ -781,6 +784,7 @@ static void init_vp_index(struct vmbus_channel *channel)
>  			break;
>  	}
> 
> +	cpus_read_unlock();
>  	channel->target_cpu = target_cpu;
> 
>  	free_cpumask_var(available_mask);
> --
> 1.8.3.1

This patch was motivated because I suggested a potential issue here during
a separate conversation with Saurabh, but it turns out I was wrong. :-(

init_vp_index() is only called from vmbus_process_offer(), and the
cpus_read_lock() is already held when init_vp_index() is called.  So the
issue doesn't exist, and this patch isn't needed.

However, looking at vmbus_process_offer(), there appears to be a
different problem in that cpus_read_unlock() is not called when taking
the error return because the sub_channel_index is zero.

Michael